1. Field of the Invention
The invention relates to a locking cover assembly for tank domes of the type used on tank trucks which transport gasoline and other inflammable materials.
2. Description of the Prior Art
It is necessary for tank domes for tanker trucks to rapidly vent the tank upon development of pressures exceeding a predetermined value.
The tank dome, which covers the filling opening in the tank, normally projects above the tank and is surrounded by a guard rail to protect it from damage, particularly if the truck should overturn in an accident. It is desirable for various reasons to keep the height of the projecting dome as low as possible.
A conventional tank dome includes an annular base adapted for engagement upon the upper edge of a cylindrical ring welded within an opening in the tank. The liquids are introduced through the central opening in the base to fill the tank. The safety locking assembly of the present invention is adapted to be mounted to the tank dome to permit the desired venting and filling operations.
State and Federal regulations require that a tank dome be capable of adequately venting a tank to protect it from the effects of both vacuum and excessive pressures. Consequently, the conventional tank dome includes both a vacuum relief valve and a pressure relief valve in the cap, each of the valves being biased to a closed position by a compression spring or the like. Upon development of a vacuum of predetermined magnitude in the tank, the vacuum relief valve automatically opens to vent air into the tank. Similarly, the pressure relief valve vents air from the tank to the outside whenever pressures develop in the tank above a predetermined value.
The vacuum and pressure relief valves are sufficient to handle moderate pressure drops and rises in the tank. For higher pressure rises, the conventional dome mounts the cap in such a way that the cap itself can be opened for venting. More particularly, the cap is mounted to a bridge element which is pivotally carried by the base. The cap mounting mechanism includes a bias element or spring which urges the cover into its seated or closed position but which permits the cap to rise against the bias of the spring to vent extraordinary pressures in the tank. This venting action normally occurs during a fire, explosion or other emergency condition. Unfortunately, the venting capacity of the cap is limited by the size of the cap, and the upward travel of the cap is in turn limited by the space available between the base and the bridge element upon which the cap is mounted. Consequently in order to increase the venting capacity of the cap to conform to government specifications, some manufacturers have raised the bridge element to provide more room for upward travel of the cap. However, the added height of the dome structure above the tank means that the height of the surrounding guard rail also has to be increased, which is undesirable for various reasons.
Another system for increasing the capacity for venting a tank is disclosed in my U.S. Pat. No. 3,365,096, issued Jan. 23, 1968, and entitled TANK CLOSURE ASSEMBLY. In that patent a base is described which is spring mounted to the peripheral ring which is welded to the tank. The base, which mounts the cover assembly, is adapted to move upwardly relative to the peripheral tank ring to vent excessive pressures developed in the tank during an emergency. Thus, the capability of the base for such upward movement compensates for the inability of the cover to move upwardly, the cover movement being limited by the space available between the base and the overlying cover bridge or lock which normally maintains the cover in its pivotally closed position.